Composition

Composition is the creation of new classes, by including other classes, and delivering a specific interface over them. It expresses a has-a relationship, as opposed to inheritance’s is-a relationship: a class holds references to other objects as properties, and delegates part of its behavior to them, rather than inheriting it from a parent class. This favors loose coupling, since the composing class can expose only the methods it chooses, while the composed objects can be swapped, reused, or tested independently. Favor composition over inheritance is a well-known object-oriented design guideline, because it avoids the rigid, fragile class hierarchies that deep inheritance chains tend to produce.

<?php

class Wheel {
    public int $diameter;
}

class GasTank {
    public int $capacity;

    public function fillUp(int $amount) {
        $this->amount = $amount;
    }

    public function empty() {
        $this->amount = 0;
    }

}

// Car composes 2 other classes: GasTank and Wheel
// These object should be accessed via the Car class, not the direct objects
class Car {
    public Wheel $wheels;
    public GasTank $gasTank;

    public function fillUp(int $amount) {
        $this->gasTank->fillUp($amount);
    }

    // empty() the gasTank is now available via the car (just drive it, or and it to garage)
}

?>

Documentation

See also Composition Over Inheritance in PHP and Object composition in PHP with Example.

Related : Inheritance, Interoperability